Fix wrong assignation of invited_by field on invitation creation process (fix #958)

Additionally it replaces the hack (django < 1.7) of double foreign key
on intermediate table (n2m relation) with trully foreign key (django 1.7
now supports it) adding corresponding migrations for conseve all previous
data.
Increment size of username and email field of user and add unique=True to email.
Merge pull request #68 from taigaio/django1.7-step2-bugs

Fix wrong assignation of invited_by field on  invitation is created.